1. Field of the Invention
This invention relates to a conductive member having at least one cover layer on a support, and a process cartridge and an electrophotographic apparatus which have a charging means having the conductive member as a charging member.
2. Related Background Art
In image-forming apparatus employing an electrophotographic system, i.e., an electrophotographic apparatus, conductive members are used as members such as charging members, developing members, transfer members and so forth. The conductive members used for such purposes are disposed in contact with, or in proximity to, an electrophotographic photosensitive member, and a direct-current voltage on which an alternating-current voltage has been superimposed is applied or only a direct-current voltage is applied to the conductive member when used.
Where the direct-current voltage on which an alternating-current voltage has been superimposed is employed as the applied voltage, a high-voltage alternating-current power source is required. This brings about an increase in the cost of the electrophotographic apparatus. Also, a large quantity of alternating currents are used, and hence the durability of the conductive members and the electrophotographic photosensitive members may decrease. Accordingly, taking account of the cost reduction and high durability of electrophotographic photosensitive members, it is preferable for the applied voltage to be only the direct-current voltage.
Meanwhile, the shape of the conductive members disposed in contact with, or in proximity to, an electrophotographic photosensitive member may include the shape of a roller, the shape of a blade, the shape of a brush, the shape of a belt, the shape of a film, the shape of a sheet and the shape of a chip. Those having the shape of a roller (that is, e.g., charging rollers, developing rollers and transfer rollers) are in wide use.
In recent years, as computers and their peripheral equipment have become popular and have been made to have high performance, electrophotographic apparatus used as output apparatus of these devices are also required to be made to have a higher function. For example, there is a trend toward color-image formation and an increase in graphic-image formation. In such a case, it is required to achieve much higher image quality and it becomes important for images to be faithfully reproduced. As one of means for dealing with these, there is a trend toward making the resolution higher. That is, original images need to be minutely recognized and reproduced, where technical development from 600 dpi toward 1,200 dpi or more is an example thereof.
Where conventional conductive members are used in such electrophotographic apparatus required to achieve much higher image quality (higher resolution), it has come about that white or black fine lines or dots appear under specific conditions or depending on the combination of conditions, such as the voltage to be applied, the environment in which images are reproduced, the patterns to be reproduced and the electrophotographic apparatus used, or that density unevenness occurs because of adhesion of foreign matter to the surfaces of conductive members or partial non-uniform adhesion of foreign matter.
In addition, with a general increase in images reproduced, it has become required for the electrophotographic apparatus to be made more highly durable than ever. In this case, the above density unevenness due to adhesion of foreign matter or partial non-uniform adhesion of foreign matter must be kept from occurring to a certain extent or less over a long period of time as a matter of course, and the conductive members themselves are also required to have high durability. At the same time, it is important to prevent the conductive members from having any negative influence on electrophotographic photosensitive members.
To solve these problems, studies have been conducted on how to prevent or lessen the adhesion or non-uniform adhesion of foreign matter, as exemplified by techniques of controlling the surface shape, the coefficient of friction or surface wettability of conductive members, and the conductive members so made up that fine particles have been made to adhere to their surfaces in advance. Such studies have achieved a certain effect.
Japanese Patent Applications Laid-open No. 2000-39755 and No. 2001-209235 also disclose a conductive member having a single-layer (a layer of a high polymer with conductive fine particles dispersed therein) structure and in which the conductive fine particles have a lower distribution density at the contact part (the surface) and in the vicinity thereof, brought into contact with a contact object member, than at other parts thereof to control the electrical resistance of the conductive member and at the same time to prevent the surface of the electrophotographic photosensitive member from being scratched by any conductive fine particles which may otherwise come off as a result of wear, or prevent the surface layer from peeling. According to this conductive member, the effect of preventing current leakage can also be obtained, and hence, suggests that the surface of this conductive member have a high electrical resistance.
At present, electrophotographic apparatus are required to be adaptable to various kinds of media (recording media) as added value, presupposing that the apparatus are made to have high-quality and high-durability. Such adaptation to media is meant to afford good image quality on various kinds of transfer materials.
At present, in offices as a matter of course and also in private use, there are increasing occasions to output data from computers in color images or graphic images. For example, in offices, a trend toward full-color printing from conventional black-and-white or monochromatic printing is rapidly being put forward. In particular, in making presentations, full-color images are preferable in view of their visual qualities and also in view of the impression the images make. In this case, images are often formed on transmitting PET films (OHT: overhead projection transparent film) as transfer materials.
Image data input devices are also rapidly evolving. For example, there are increasing occasions to i) photograph electronic pictures with digital cameras and input them into computers to perform image processing or editing as the occasion requires, to output the data by means of printers, or ii) copy photographs directly by means of copying machines. In the case when photographic image data are outputted, specialities (speciality paper) (e.g., surface-treated paper and high-gloss paper) are often used as transfer materials. The OHTs and specialities are thicker than plain paper and also differ in materials from plain paper in some cases. In order to form good images on such transfer materials, the process speed is in some cases made lower than that in using plain paper, to adapt reproduction to this material.
In private use also, for example, not only are the specialities used in some cases, but also thick and small-size sheets, such as postcards, are frequently used.
Thus, in order to perform an adaptation to such media (transfer materials) which are different with respect to materials, thickness and size, it is preferable that one electrophotographic apparatus can output image data at a plurality of different process speeds so that proper speeds can be set correspondingly thereto. For example, it is the case that the apparatus is so constructed that a plurality of different process speeds, such as regular speed and ½ speed, ⅓ speed and ¼ speed of the regular speed can be set, where, e.g., the apparatus is used at 94 mm/s (regular speed) in the case of plain paper and at 31 mm/s (⅓ speed) in the case of OHTs.
However, such differences in process speed have a great influence on image uniformity, as so revealed as a result of studies.
Where conventional conductive members are used, especially used as charging members, in such electrophotographic apparatus that can set a plurality of different process speed in one machine, the following problem may arise.
In the case of an electrophotographic apparatus having employed the system in which only direct-current voltage is applied to the conductive member as a charging member, even a charging member which can achieve good charging uniformity at, e.g., 94 mm/s (regular speed) may cause fine and short, white or black horizontal lines at, e.g., 31 mm/s (⅓ speed). This phenomenon tends to appear especially in a low-humidity environment. It has also been found that such white or black horizontal lines may greatly differ depending on the construction of the electrophotographic photosensitive members.
In the case of an electrophotographic apparatus having employed the system in which a voltage formed by superimposing an alternating-current voltage on a direct-current voltage is applied to the charging member, the charging uniformity can be dealt with by appropriate selection of the frequencies of the alternating-current voltage according to the process speed. However, the current leakage tends to occur especially on the low-speed side. This phenomenon tends to appear especially in a high-humidity environment.
Where the conductive member disclosed in Japanese Patent Applications Laid-open No. 2000-39755 and No. 2001-209235 is used, which has a single-layer (a layer of a high polymer with conductive fine particles dispersed therein) structure and in which the conductive fine particles have a lower (or substantially zero) distribution density at the contact part (the surface) and in the vicinity thereof, brought into contact with a contact object member, than at the other part thereof to control the electrical resistance, the following problem may also arise.
The conductive fine particles have the effect of lowering electrical resistance and at the same time have reinforcing properties. The fact that the conductive fine particles have a lower distribution density as they come in the vicinity of the contact part means that the layer has a smaller quantity of the conductive fine particles in the vicinity of the surface. As the result, the layer is less reinforced (has a lower strength) or has a lower hardness closer to the surface. This applies all the more when the quantity of the conductive fine particles is substantially zero.
More specifically, in this construction, the layer has a low hardness or a low strength at the surface and in the vicinity thereof, and hence the surface and the vicinity thereof are in the state in which they can wear easily.
To deal with this effect, a thickness of about 20 μm is substantially necessary as the lower limit. This, however, means that the matter is dealt with by controlling the thickness without overcoming the easiness of wear, and can not safely be said to be a fundamental improvement.
In particular, in the case of the electrophotographic apparatus that can set a plurality of different process speed in one machine, not only the static or dynamic state of contact, the torque, the state of rubbing friction, the state of application of voltage, and so forth, between the electrophotographic photosensitive member and the conductive member may change irregularly, but also how they correlate with each other may differ to an extent. Hence, various stresses tend to be applied to a greater extent than the case of the electrophotographic apparatus having a single process speed. As the result, the influence of such external factors on conductivity may become complicated and also the surface of the conductive member tends to wear more. This is very remarkable with rubbers.
Thus, although the conductive fine particles can be made to come off less because of wear of the surface of the conductive member, the surface itself may wear earlier and hence it follows that the performance at the initial stage is lost in a short time. In this regard, the above measures are unsuitable and insufficient for making the conductive member itself highly durable.
Moreover, if the surface and the vicinity thereof has worn to become lost, the conductive fine particles become bare from the interior, and hence the problem caused by the coming off of the conductive fine particles may arise. Also, the larger the thickness of the part where the quantity of the conductive fine particles is substantially zero, the more unfavorable it is for the charging uniformity of charging the electrophotographic photosensitive member uniformly and the more faulty images tend to occur. This tendency is remarkable in the electrophotographic apparatus in which only direct-current voltage is applied to the conductive member for charging of the electrophotographic photosensitive member.